(a) Field of the Invention
The present invention relates to an infrared ray detector having a vacuum encapsulation structure and, more particularly, to an improvement of an infrared ray detector in the vacuum encapsulation structure. The present invention also relates to a method for fabricating such an infrared ray detector.
(b) Description of the Related Art
A thermo-sensitive infrared Tay detector (hereinafter referred to as simply infrared ray detector) has a function for absorbing infrared ray irradiated from an object to convert the same into heat by using an infrared-ray receiving film having an optical resonator structure. The heat converted from the infrared ray raises the thermo-sensitive resistor such as a bolometer film constituting a diaphragm having a micro-bridge structure to change the resistance of the resistor, allowing the temperature of the object to be determined based on the change of the resistance.
FIG. 41 shows the structure of a conventional infrared ray detector, which includes a plurality of pixels each having an infrared ray receiving section (or photoreceptor section) 19 formed by a plurality of protective films 5, 7 and 9, such as made of silicon nitride, for absorbing incident infrared ray and a bolometer film 6 sandwiched between the protective films 5 and 7. The photoreceptor section 19 is supported in spaced relationship with a substrate 1 including a read-out circuit by metallic interconnects etc. constituting a beam structure 18 to be thermally isolated from the substrate 1. Infrared ray incident onto the photoreceptor section 19 raises the temperature of the photoreceptor section 19 to change the resistance of the bolometer film 6 as one of thermo-sensitive materials constituting the photoreceptor section 19.
The infrared ray detector as described above is received in a vacuum package, whereby a hollow section, or vacuum space, 20 is maintained at a vacuum to intercept the heat flowing from the photoreceptor section 19 to the underlying substrate 1 and the heat radiation from the photoreceptor section 19 is effected only by the beam 18. By reducing the heat conductivity of the beam 18, an excellent heat isolation structure can be obtained between the photoreceptor section 19 and the substrate 1.
FIG. 42 shows a conventional technique for receiving the infrared ray detector in a vacuum, wherein the wafer on which a plurality of infrared ray detectors are formed is separated into a plurality of chips, each of which is bonded onto a package 21 and encapsulated by a cap member 23 having a window 24 for passing therethrough the incident infrared ray. Before the encapsulation step, each chip is subjected to a wire bonding process wherein the electrodes of the infrared ray detector are bonded onto external pins 22, and then subjected to a evacuation process using an exhaust tube 25.
The procedure for obtaining the infrared ray detector as described above is complicated and not suited to a mass production due to the following reasons. First, since the wafer on which the infrared ray detector array is fabricated has a weak mechanical surface due to the micro-bridge structure thereof, the dicing step for separation into a plurality of chips and the bonding step for each chip should be performed by manual operation, which raises the fabrication costs of the infrared ray detectors.
Second, the evacuation process which involves coupling of each of the packages to an evacuation device by using the exhaust tube is performed for the each of the packages to raise the man-hours for the infrared ray detector, although the plurality of chips are formed at once on a wafer by a series of processes.
Patent Publication JP-A-9-506712 effected for a PCT application describes a vacuum encapsulation process conducted in a wafer level of the infrared ray detectors. The process roughly includes the steps of fabricating a plurality of infrared ray detectors on a wafer, bonding together the wafer and another wafer, on which a plurality of windows are formed, in a vacuum ambient, encapsulating the bonded wafers in the vacuum ambient, and separating the wafer into a plurality of chips.
The process as described in the publication may reduce the costs for man-hours and materials, and also reduce the dimensions of the infrared ray detector modules. However, the process requires a double-surface exposure system for additionally treating the another wafer having the windows as well as a particular wafer-bonding system for bonding together both the wafers with superior dimensional accuracy. Use of these systems also increases the fabrication steps and requires larger man-hours. In addition, the process including a vacuum encapsulation step by using a molten solder material having a large area makes it difficult to maintain the degree of vacuum due to exhaust of the inner gas received in the solder material.
Patent Publication JP-A-11-326037 describes a fabrication process for infrared ray detectors, such as shown in FIG. 43, which includes the steps of bonding together a silicon wafer 26, on which a plurality of infrared ray detectors are formed, and another silicon wafer 27, on which a plurality of windows are formed, in an atmospheric ambient, and evacuating the internal of the bonded wafers by using a through-hole 31 formed in the silicon wafer 27. The process described in this publication has an advantage in that the evacuation step effected after bonding the wafers assures a higher degree of vacuum.
However, the process described in JP-A-11-326037 also requires a particular wafer-bonding system, and in addition, it is difficult to form the through-hole 31 beforehand in the thick wafer 27 mounting thereon the windows and having a large thickness of several hundreds micrometers. Further, the process requires two melting steps for solders, including the steps of melting the bonding solder in the bonding process and melting the vacuum-encapsulation solder in the vacuum encapsulation process. The two melting steps increase the man-hours and thus raise the costs for the infrared ray detectors. Further, it is difficult to maintain a high degree of vacuum in the bonded wafers due to the limited property of the solder, thereby degrading the long-term reliability of the infrared ray detector.